1. Field of the Invention
The present invention relates to a technology for processing an image captured by a fisheye optical system having a wide angle and a large chromatic aberration of magnification.
2. Description of the Related Art
Recently, demands for a wide-angle imaging apparatus are increasing for applications such as a back monitor of a vehicle. However, as the angle becomes wider, the chromatic aberration of magnification and distortion become large, making it difficult to design an optical system with a small aberration. Therefore, performance needs to be improved in combination with image processing.
In the imaging apparatus using the optical system having the chromatic aberration of magnification and the distortion, a conventional technique for correcting a distortion includes a method of correcting the chromatic aberration of magnification and the distortion at the same time by performing coordinate transformation independently for each color component of red (R), green (G), and blue (B) with respect to an R-signal, a G-signal, and a B-signal obtained by the imaging device such as a charge coupled device (CCD) or a complimentary metal oxide semiconductor (CMOS) sensor at a subsequent stage, as described in Japanese Patent Application Laid-open No. 2006-345054. Also, there is a method of correcting only the distortion by performing coordinate transformation for respective color components of RGB together, ignoring the chromatic aberration of magnification.
At the time of correcting the distortion, for example, as described in Deguchi, “A Survey of Recent Camera Calibration Techniques”, Information Processing Society of Japan, Study group material CV-82-1, 1993, there has been proposed a method of correcting a distortion based on following equations:X=x+Kx(x2+y2)×x Y=y+Ky(x2+y2)×y, where it is assumed that a destination of coordinate transformation (original coordinate) is (x, y), a source of coordinate transformation (coordinate at a destination due to aberration) is (X, Y), and parameters determining a distortion amount in respective directions of x and y are Kx and Ky.
Although the distortion is uniform in respective color components of RGB, the chromatic aberration of magnification is different for each color component. Further, the chromatic aberration of magnification is smaller than the distortion. Therefore, it is desired to correct the chromatic aberration of magnification and the distortion separately.
According to the conventional method of simultaneously correcting the chromatic aberration of magnification and the distortion by performing coordinate transformation independently for each color component of RGB, a memory having a large capacity and a small latency at the time of random access such as a static random access memory (SRAM) or a multiport memory is required for handling respective colors of RGB. However, the large-capacity SRAM and the memory having a plurality of ports are very expensive, and particularly in the case of high resolution requiring a large-capacity memory, the apparatus becomes even more expensive.
Meanwhile, the method of correcting only the distortion by commonly performing coordinate transformation for respective colors of RGB requires a large-capacity memory, but an inexpensive dynamic RAM (DRAM) having a one-chip configuration can be used. However, if the case is with a wide angle in which the chromatic aberration of magnification cannot be ignored, this method cannot be applied.
An independent correction circuit is required for each color component for correction of the chromatic aberration of magnification, and correction of the chromatic aberration of magnification needs to be performed in the minimum circuit size, to reduce the cost. However, because a complicated polynomial or the like is generally used as a formula of coordinate transformation, there is a problem in that the circuit size increases. For example, according to the method described in “Recent Trend of Camera Calibration Method”, because a quadratic term or more is included, an analog multiplier is required for calculation. Further, in the case of the fisheye optical system having a large chromatic aberration of magnification, the analog multiplier requires a large-scale circuit, because a large number of bits are required due to a problem of calculation accuracy.
The fisheye optical system refers to an optical system that executes projection methods such as stereographic projection y=2f tan(θ/2), equidistant projection y=fθ, equisolidangle projection y=2f sin(θ/2), and orthogonal projection y=f sin θ other than a projection method in a normal lens y=f tan θ, where an image height is y, a focal length is f, and a half view angle is θ.